Process control systems require the accurate measurement of process variables. Typically, a primary element senses the value of a process variable and a transmitter develops an output having a value that varies as a function of the process variable. For example, a level transmitter includes a primary element for sensing level and a circuit for developing an electrical signal proportional to sensed level.
Knowledge of level in industrial process tanks or vessels has long been required for safe and cost-effective operation of plants. Many technologies exist for making level measurements. These include buoyancy, capacitance, ultrasonic and microwave radar, to name a few. Recent advances in micropower impulse radar (MIR), also known as ultra-wideband (UWB) radar, in conjunction with advances in equivalent time sampling (ETS), permit development of low power and lost cost time domain reflectometry (TDR) instruments.
In a TDR instrument, a very fast pulse with a rise time of 500 picoseconds, or less, is propagated down a probe, that serves as a transmission line, in a vessel. The pulse is reflected by a discontinuity caused by a transition between two media. For level measurement, that transition is typically where the air and the material to be measured meet. These instruments are also known as guided wave radar (GWR) measurement instruments.
In addition to detecting the upper surface of a liquid, which is actually an air-liquid interface, the GWR instrument has the ability to measure the location of the interface between two immiscible liquids of differing density and dielectric properties, such as oil over water. Provided that the upper layer is sufficiently thick, and under appropriate conditions, the analog wave form will exhibit two pulses. One pulse is at the liquid surface and a second pulse is at the interface between the two liquids. However, this technique fails when the media produces only one discernible pulse. This can occur when the media consists of only one material or when the upper layer is too thin to produce a distinct pulse, i.e., the pulses from the upper surface and the interface merge into one. When this occurs in practical applications, it is important for the instrument to distinguish which material is present, such as oil or water.
Known GWR instruments approach the issue of distinguishing oil from water, given a single pulse, strictly from a signal amplitude perspective. If the signal strength of the pulse is less than a given threshold, then the pulse is assumed to be from oil. Otherwise, it is assumed to be from water. The concern with this approach is that changes in process conditions and sensitivity can affect accuracy.
The present invention is directed to solving one or more of the problems discussed above in a novel and simple manner.